The present invention utilizes the principles of Euler's Theorem which states that when a rigid body moves in such a manner that one point, O, remains fixed in space, it can be shown that the resulting displacement is a rotation about some axis through O.
The asymmetric flow within a rocket's nozzle will produce a turning moment on the body. In free rocket applications, the target miss distance can be correlated to the magnitude of this parameter, i.e. thrust malalignment. It has been commonly accepted within this discipline that it is attached or fixed to the body coordinate system and has been treated accordingly in the analyses. Designers have imposed severe mechanical tolerances on the metal parts to assure that this parameter is within an acceptable limit, less than one milliradian for most free rockets. Often the propulsion system specification treats this source empirically and provides acceptance criteria via inspection of metal parts in a quasistatic environment. This practice has been adopted because the measurement standards are insensitive to the side thrust produced. In other words, the delivered side impulse is submerged within the noise level and it is not readily discernable.
A quality measurement of this parameter has eluded designers and test engineers. Significant resources have been expended just to investigate this source. Likewise, many concepts have been evaluated by industry and governmental agencies. These concepts have included 2.degree., 3.degree. and 6.degree.-of-freedom test stands and most users conclude that a 1 mil measurement precision is highly unlikely. This uncertanity is predominantly caused by cross component frequency coupling. This frequency phenomena is caused by inherent deflection of load strut assemblies which do not register a signal without an attendant deflection. Best results have been obtained when the load struts were placed in a set of orthogonal axes. Typically, each load strut will deflect 0.010-0.020 inch under full scale design load, and each strut contains a pair of flexures to assure only axial loading of the sensor. Unfortunately, the pitch, yaw, and roll frequencies will be near the same quantity. There are three common problems with this type of a test stand and are as follows:
1. Alignment of the test specimen to the reference coordinate system, PA1 2. Calibration required to define the interaction coefficients of the system, and, PA1 3. Frequency cross coupling between the components.
The rocket's ignition sequence usually excites the spring mass system such that the actual side thrust (malalignment) which is contained within the frequency spectrum is not discernible. This problem is magnified with short burn time and high thrust motors, especially, when there is little damping with the system. Data assessment is extremely complex and often the evaluator is left with much latitude during interpretation.